Frame structure for a QAM system

ABSTRACT

A novel framing method for a variable net bit rate digital communications system that utilizes a set of different QAM constellations and punctured trellis code combinations, each combination designated as a mode. This frame structure has a variable integral number of QAM symbols per frame depending on the selected mode, but the number of bytes and Reed-Solomon packets per frame is constant. This is achieved even though the number of data bits per QAM symbol for some modes is fractional. Also the number of trellis coder puncture pattern cycles per frame is an integer for all modes. This arrangement simplifies the synchronization of receiver processing blocks such as the Viterbi decoder, de-randomizer, byte de-interleaver, and Reed-Solomon decoder.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application is continuation of U.S. patent application Ser.No. 13/929,567 filed Jun. 27, 2013, entitled “Frame Structure For A QAMSystem,” which was a continuation-in-part of U.S. patent applicationSer. No. 13/229,596 filed Sep. 9, 2011, entitled “Systems And MethodsFor Detecting Tampering With Video Transmission Systems,” which was acontinuation in-part of U.S. Provisional Patent Application No.61/495,287 filed Jun. 9, 2011, U.S. patent application Ser. No.12/698,041 filed Feb. 1, 2010, U.S. patent application Ser. No.12/698,066 filed Feb. 1, 2010 which issued as U.S. Pat. No. 8,374,270,U.S. patent application Ser. No. 12/698,071 filed Feb. 1, 2010 whichissued as U.S. Pat. No. 8,428,188, U.S. patent application Ser. No.12/698,037 filed Feb. 1, 2010 which issued as U.S. Pat. No. 8,369,435,and U.S. patent application Ser. No. 12/698,061 filed Feb. 1, 2010 whichissued as U.S. Pat. No. 8,422,611, all of which applications are herebyexpressly incorporated by reference herein.

The present Application is related to U.S. patent application Ser. No.12/363,669, filed Jan. 30, 2009, entitled “Mixed Format MediaTransmission Systems and Methods,” which issued as U.S. Pat. No.8,300,114 and which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to digital communicationssystems and more particularly to framing in digital communicationssystems.

2. Description of Related Art

Framing in Digital Communications Systems

Digital data streams generally have some sort of frame structure suchthat the data is organized into uniformly sized groups of bits or bytes.Any system that uses block based forward error correction (FEC) willhave frames organized around the error correction code word size. Also,if the system uses interleaving to combat impulse noise, the framestructure will be arranged with the interleaver parameters in mind. Ifthe system uses data randomization to achieve a flat spectrum, thepseudo-random sequence utilized may be synchronized to the framestructure, restarting at the beginning of each frame.

For an RF digital communications system, a receiver must typically firstachieve carrier and symbol clock synchronization and equalization. Itcan then recover the transmitted data. But, to make sense of thisincoming data stream, the receiver must also synchronize to the framestructure. In other words, the receiver must know where the errorcorrection code words start and end. It also must be able to synchronizereceiver modules such as the deinterleaver to match the interleaveroperation of the transmitter so that the resultant deinterleaved bits orbytes are correctly ordered, and the de-randomizer to match the startingpoint of the pseudo-random sequence used in the transmitter to flattenthe spectrum.

Conventional systems often provide for receiver frame synchronization byappending a known pattern of symbols of a fixed length at the beginningor end of the frame. This same pattern repeats every frame, and it oftenconsists of a 2 level (i.e. binary) pseudo-random sequence withfavorable auto-correlation properties.

Examples of Frame Structure in Existing Systems

With reference to FIG. 1, the ATSC digital television (DTV) terrestrialtransmission standard adopted in 1996 provides a system in which data istransmitted in frames. Each frame 13 is composed of 313 segments, andeach segment contains 832 symbols for a total of 260,416 symbols perframe. The first four symbols in each segment are segment sync symbols12 comprising the sequence [+5, −5, −5, +5]. The first segment in eachframe is a frame sync segment 14 with 312 data segments 16, 18.Referring now to FIG. 2, frame sync segment 14 has a segment sync 200, a511 symbol pseudo-random noise (PN511) sequence 202, a 63 symbolpseudo-random noise (PN63) sequence 204, a second PN63 sequence 206 anda third PN63 sequence 208. This is followed by 24 mode symbols 213indicating that the mode is 8 VSB. Pre-code symbols 214 and reservedsymbols 212 complete frame sync segment 14.

The segment sync 200 and PN511 202 symbols are a priori known to thereceiver and may be used to acquire frame synchronization viacorrelation methods. All of the aforementioned symbols come from the set{+5, −5}. The last 12 symbols of this segment are from the set {−7 −5 −3−1 +1 +3 +5 +7}, and are duplicates of the last 12 symbols of thepreceding data field. These are called the precode symbols (notdiscussed here).

Referring also to FIG. 3, for each of the subsequent 312 segments of thefield, referred to as data segments, the 828 symbols 32 following thefour segment sync symbols 30 are created from a single 207 byte (1656bit) Reed-Solomon (RS) code-word by taking 2 bits at a time, trellisencoding them into 3 bits, then mapping each unit of 3 bits to an 8level symbol from the set {−7 −5 −3 −1 +1 +3 +5 +7}.

Another example of framing in a digital communications system is seen inthe ISDB-T system. Unlike the single-carrier ATSC system, ISDB-T is amulti-carrier system utilizing coded orthogonal frequency divisionmultiplexing (“COFDM”). For example, mode 1 for ISDB-T uses 1404carriers. A frame consists of 204 COFDM symbols and each COFDM symbolcan be thought of as a combination of 1404 independent QAM symbols, onefor each of the carriers. Thus, the frame is composed of a combinationof 204×1404=286416 QAM symbols. Of these, 254592 are data, and 31824comprise both pilot information (which can be used for framesynchronization) and mode information which are scattered throughout theframe in a known pattern. A simplified view of this frame arrangement isshown in FIG. 4. It can be seen that the pilot and mode information isscattered about the frame in a known pattern.

This system has modes that utilize three different QAMconstellations—QPSK, 16 QAM, and 64 QAM. It also supports five differenttrellis coding rates (½, ⅔, ¾, ⅚, ⅞) based on a single punctured mothercode. This well-known technique makes it very economical to construct asingle Viterbi decoder in the receiver that can easily be adjusted todecode all five of the specified codes.

Prior to trellis coding at the transmitter, the data is formed into 204byte (1632 bits) long RS blocks. While the number of COFDM symbols perframe is always constant, the number of RS blocks per frame varies withthe selected mode, but most importantly, that number is always aninteger. This allows for easy RS block synchronization in the receiveronce frame sync has been established and the trellis code rate is known.In order for this to be true, the number of data bits per frame prior totrellis coding must be evenly divisible by 1632 for all modes.

TABLE 1 Data Bits per Frame for ISDB-T data bits/frame (before trelliscoding) bits/frame after mode 1/2 2/3 3/4 5/6 7/8 trellis coding QPSK254592 339456 381888 424320 445536 509184 16 QAM 509184 678912 763776848640 891372 1318368 64 QAM 763776 1318368 1145664 1272960 13366081527552

Table 1 shows the number of data bits per frame for all the modes(combination of QAM constellation and trellis code rate). In every casethe number of data bits per frame is evenly divisible by 1632 (data bitsmeans bits before trellis coding).

BRIEF SUMMARY OF THE INVENTION

Certain embodiments of the present invention a framing structure ofmodulation systems used in digital communications systems. Inparticular, signaling systems and methods are provided that can beemployed in security systems. A convolutional byte interleaverinterleaves a frame of data, wherein the interleaver is synchronized toa frame structure and a randomizer may be configured to produce arandomized data frame from the interleaved data frame. In one example, apunctured trellis code modulator is operated at a selectable code ratethat produces a trellis coded data frame from the randomized data frame.A QAM mapper maps groups of bits in the trellis coded data frame tomodulation symbols, thereby providing a mapped frame and a synchronizeradds a synchronization packet to the mapped frame. The punctured trelliscode modulator can be bypassed as desired to obtain an optimized net bitrate under various white noise conditions, thereby permittingperformance optimization of the system.

An identical synchronization packet may be added to each mapped frameand, in one example, this packet can comprise 127 symbols. Thesynchronization packet may include different binary sequences for realand imaginary parts of the modulation symbols, although it iscontemplated that a portion of the synchronization packet comprises anidentical binary sequence for both real and imaginary parts of themodulation symbols. Moreover, the synchronization packet can comprisedata that indicates a transmission mode for the mapped frame, where thetransmission mode may include a selected QAM constellation and aselected trellis code rate. In some embodiments, the system generates aconstant integral number of Reed-Solomon packets for each frame, avariable integer number of modulation symbols for each frame, and anintegral number of puncture pattern cycles per frame of data regardlessof transmission mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a frame structure used in ATSC digitaltelevision.

FIG. 2 is an example of a conventional frame synchronization packet.

FIG. 3 is an example of a data segment in a conventional data frame.

FIG. 4 provides a simplified view of a frame arrangement.

FIG. 5 is a block schematic of a modulator according to certain aspectsof the invention.

FIG. 6 is a block representation of a frame structure employed incertain embodiments of the invention.

FIG. 7 illustrates operation of a convolutional byte interleaver incertain embodiments of the invention.

FIG. 8 is a block schematic of a selectable code rate punctured trelliscoded modulation employed in certain embodiments of the invention.

FIG. 9 illustrates examples of QAM mappings.

FIG. 10 shows a frame sync/mode packet.

FIG. 11 is a simplified frame structure employed in certain embodimentsof the invention.

FIG. 12 is a schematic of a demodulator according to certain aspects ofthe invention.

FIG. 13 is a simplified block schematic illustrating a processing systememployed in certain embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples so as to enable those skilled in the art to practice theinvention. Notably, the figures and examples below are not meant tolimit the scope of the present invention to a single embodiment, butother embodiments are possible by way of interchange of some or all ofthe described or illustrated elements. Wherever convenient, the samereference numbers will be used throughout the drawings to refer to sameor like parts. Where certain elements of these embodiments can bepartially or fully implemented using known components, only thoseportions of such known components that are necessary for anunderstanding of the present invention will be described, and detaileddescriptions of other portions of such known components will be omittedso as not to obscure the invention. In the present specification, anembodiment showing a singular component should not be consideredlimiting; rather, the invention is intended to encompass otherembodiments including a plurality of the same component, and vice-versa,unless explicitly stated otherwise herein. Moreover, applicants do notintend for any term in the specification or claims to be ascribed anuncommon or special meaning unless explicitly set forth as such.Further, the present invention encompasses present and future knownequivalents to the components referred to herein by way of illustration.

Certain embodiments of the invention provide novel frame structures fora single carrier communication system. In conventional systems, anauto-correlation of a known pattern of symbols of a fixed length at thebeginning or end of a frame at zero offset yields a large value, if theoffset is non-zero the correlation value (side-lobe) is very small.However, the correlation for this frame sync sequence with randomsymbols will yield a small value. Therefore, a receiver may execute acorrelation of incoming symbols with a stored version of the frame syncpattern in order to obtain a large value at the exact start of eachframe enabling the receiver to determine the starting point of eachframe. There can be several modes of operation for the communicationsystem. The modes can include a variety of combinations of symbolconstellations, trellis codes, and interleave patterns. The receivermust have knowledge of the mode in order to successfully recover thetransmitted data. This can be achieved by adding additional mode symbolsto the frame sync pattern. These mode symbols can be reliably receivedby using correlation methods since they are sent repeatedly every frame.They can be made even more robust by encoding them using a block code.

One frame structure according to certain aspects of the inventionutilizes punctured trellis coding and QAM constellation combinationssimilar to those used in ISDB-T. The number of symbols per frame can bea variable integer depending on the mode and the number of RS packetsper frame is a constant integer regardless of mode. This arrangementsimplifies the design of receiver processing blocks such as thede-randomizer and the de-interleaver because the number of RS packetsper frame is always fixed. In conventional systems such as ISDB-T, thenumber of symbols per frame is constant and the number of RS packets perframe is a variable integer depending on the mode. The frame will bedescribed with reference to an example depicted in FIG. 5 of atransmitter architecture that is constructed according to certainaspects of the invention.

An RS encoder 500 accepts byte data 501 and an externally generatedframe sync signal that indicates the start of each group of 315Reed-Solomon packets 522. As shown in FIG. 6, each packet 60 comprises207 bytes, of which 20 are parity bytes 62. These 315 Reed-Solomonpackets form forward error correction (“FEC”) data frame 522 whichcontains 65205 bytes.

A convolutional byte interleaver 502 follows. FIG. 7 illustrates a modeof operation of interleaver 502 that combats impulse noise affecting thetransmitted signal. The parameter B in paths 76, 78 is set to 207, andparameter M in paths 72, 74, 76 and 78 is set to 1. Frame sync signal503 forces input and output commutators 70 and 71 to the top position700, thus synchronizing the interleaving to the frame structure. Inputand output commutators 70 and 71 move down one position 702 as a byteenters the interleaver and a different byte exits the interleaver. Whencommutators 70 and 71 reach the bottom 708, they shift back to the top700. Each of the B parallel paths 706, 708 contains a shift register 76and 78 having the length shown in the FIG. 7.

A randomizer 506 produces a randomized FEC data frame 528 by operatingon the 65205×8=521640 bits of the FEC data frame 524 by executing anexclusive or operation on those bits with a PN (pseudo-random noise)sequence of length 219−1 which is shortened by resetting the PN sequencegenerator at every frame sync time.

An example of a selectable code rate punctured trellis coded modulation(“PTCM”) module 508 is shown in more detail in FIG. 8. PTCM 508 uses amethod known to those of skill in the art. The method that starts with a64 state ½ rate coder and executes puncturing to achieve any one of 5different code rates. In certain embodiments, the PTCM 508 can also becompletely bypassed (code rate=1). This allows for a selectable tradeoff between net bit rate and white noise performance for the system.Similar trellis coding techniques are used in ISDB-T and DVB-T systems.PTCM produces two bits 532 at the output for every bit provided to theinput 528. However, some of the output bits 532 are discarded accordingto the selected code rate and corresponding puncture pattern.

QAM mapper 513 takes the bits in groups of 2, 4 or 6 from the coderoutput 532 and maps them into QPSK, 16 QAM, or 64 QAM symbolsrespectively. Examples of such mappings are provided in FIG. 9.

Module 512 adds a frame-sync/mode symbol packet (all symbols are QPSK)to the start of each FEC data frame 534. With reference to FIG. 13, thefirst part 130 of this packet comprises 127 symbols and comprises anidentical binary PN sequence for both the real and imaginary parts ofthe symbols. Other PN sequence lengths are possible, and the real andimaginary parts can have the opposite sign. The second part 132 of thispacket comprises data that indicate the transmission mode—the selectedQAM constellation and the selected trellis code rate. This mode data canbe encoded using a block error correction code for added reliability atthe receiver. Methods that can be employed include BCH coding and otherblock codes. In one example, 6 possible trellis code rates includingbypass are possible. Additionally, three constellations are possibleresulting in 18 modes. Accordingly, 5 bits are needed to represent eachof the possible mode selections. The 5 bits could be encoded into a 16bit code word using an extended BCH code. Since each QPSK symbolcontains 2 bits, 8 mode symbols would be required.

FIG. 11 illustrates a frame structure 536 provided to passbandmodulation (“PB Mod”) module 514. Packets 113 comprise 315 RS packets(521640 bits). The number of QAM symbols to which the 315 RS packets 113are mapped can vary with the mode selection. The PB Mod module 514 thenmodulates the baseband QAM symbols to passband using any suitable methodknown to those with skill in the art.

Frame structures according to certain aspects of the inventionadvantageously overcome certain shortcomings and failings ofconventional frames. In particular, the frame structure offers for allmodes:

-   -   a constant integral number of RS packets per frame regardless of        mode, and    -   the number of QAM symbols per frame is a variable integer for        all modes    -   an integral number of puncture pattern cycles per frame for all        modes        Note that providing an integer number of QAM symbols per frame        is not a trivial accomplishment because the FEC data frame must        exactly comprise I×207 data bytes where I is a selected integer        in order to have a fixed integral number of RS packets per        frame. Accordingly, the number of data bits per frame prior to        trellis coding must not only be an integer, but the number must        be evenly divisible by 207×8=1656 for all modes. Furthermore,        the number of trellis coder output bits per QAM symbol is 2, 4        and 6 bits respectively for QPSK, 16 QAM and 64 QAM (See        Table 2. which shows a code rate=1 for trellis code bypass).        Additionally, trellis coding adds bits. The number of data bits        per symbol prior to trellis coding is shown in Table 2, where        each entry is calculated as:        right-most column entry/code rate

TABLE 2 Data Bits per Symbol (input bits to trellis coder per mapped QAMsymbol) trellis code rates constellation 1/2 2/3 3/4 5/6 7/8 1 QPSK 1.004/3 1.50  5/3 1.75 2.00 16 QAM 2.00 8/3 3.00 13/3 3.50 4.00 64 QAM 3.004.00 4.50 5.00 5.25 6.00

The fact that the number of data bits per symbol can be fractionalrequires that the RS packet size and the number of RS packets per framebe precisely selected. With RS packet size of 207 and 315 packets perframe an integral numbers of symbols per frame is attained. As shown intable 3, each entry can be calculated as:number of data bits per frame/number of data bits per symbol=521640/entry from table 2

TABLE 3 Symbols per Frame trellis code rates constellation 1/2 2/3 3/45/6 7/8 1 QPSK 521640 391230 347760 312984 298080 260820 16 QAM 260820195615 173880 156492 149040 130413 64 QAM 173880 130413 115920 13432899360 86940

This frame provides the additional advantage that there are an integralnumber of puncture pattern cycles per frame (pp/frame) for all modes. Inorder to correctly decode the punctured trellis coded data, the decoderin the receiver must know how the puncture pattern aligns with the data.The bit-wise puncture patterns applied at the output of the mother codetrellis coder are indicated in the second column of the table in FIG. 8.The number of 1's in each puncture pattern is the puncture patternlength. In the proposed system the puncture pattern always lines up withthe start of the FEC data frame. This allows the use of frame sync inthe receiver to properly align the de-puncturer in the receiver Viterbidecoder with the bit stream. The desired alignment is indicated in Table4 which shows an integral number of pp/frame for all modes. The puncturepattern per symbol (“pp/symbol”) entries can be calculated as:pp length/# of trellis coder output bits per symbol

The pp/frame entries can be calculated as:symbols per frame from table 3/pp/symbol

TABLE 4 Puncture Patterns per Frame QPSK 16 QAM 64 QAM (2 bits/sym) (4bits/sym) (6 bits/sym) code pp pp/ pp/ pp/ pp/ pp/ pp/ rate lengthsymbol frame symbol frame symbol frame 1/2 2 1 521640 2 521640 3 5216402/3 3 2/3 260820 4/3 260820 2 260820 3/4 4 1/2 173880 1 173880 3/2173880 5/6 5 1/3 134328 2/3 134328 1 134328 7/8 8 1/4 74520 1/2 745203/4 74520 1 NA NA NA NA NA NA NA

It will be appreciated that other combinations of RS packet sizes andnumbers of packets per frame can be used to obtain the same desiredresult. The numbers provided herein are described for purposes ofillustration only.

As shown in FIG. 12, certain embodiments of the invention provide areceiver architected to handle a frame structured according to certainaspects of the invention. Module 1200 receives and converts transmitteddata in a passband signal to baseband QAM symbols. The operationsperformed by module 1200 can include symbol clock synchronization,equalization (to remove inter-symbol interference) and carrier recovery,typically using sub-modules. Accordingly, module 1200 may comprise anequalizer that outputs recovered baseband QAM symbols 1201. Baseband QAMsignals 1201 are provided to two-level slicer 1218 for slicing in boththe real and imaginary directions, thereby forming the sequencesa_(R)[k]ε[−1,+1] and a₁[k]ε[−1,+1] 1219 which are provided to frame-syncmodule 1220.

Frame sync module 1220 performs a continuous cross-correlation operationon the incoming sliced QAM symbols 1219, separately for both the realand imaginary parts, with a stored copy of the binary frame-sync PNsequence. Each member of the stored copy has a value of −1 or +1. Thisoperation is given by:

${b_{R}\lbrack k\rbrack} = {{\sum\limits_{n = 0}^{126}\;{{s\lbrack n\rbrack}{a_{R}\left\lbrack {n - k} \right\rbrack}\mspace{14mu}{and}\mspace{14mu}{b_{I}\lbrack k\rbrack}}} = {\sum\limits_{n = 0}^{126}\;{{s\lbrack n\rbrack}{a_{I}\left\lbrack {n - k} \right\rbrack}}}}$where s is the stored copy in the 127 long frame-sync PN sequence. Themaximum magnitude of either b_(R) or b_(I) indicates the start of theFEC data frame.

Once the frame sync start position is located, the position of the codewords containing the mode bits (constellation and trellis code rate) isknown. The code words can be reliably decoded by, for example, a BCHdecoder or by correlating the received code word with all the possiblecode words and choosing the code word yielding the highest resultingvalue. Since this information is sent repeatedly, additional reliabilitycan be obtained by requiring that the same result occur multiple timesbefore it is accepted.

This derived frame-sync signal 1221 is used to indicate which symbolsare to be removed in “remove frame-sync/mode symbols” module 1204 beforesymbols are fed to soft de-mapper 1206. In one example, 127 frame-syncsymbols and 8 mode symbols are removed from the stream ensuring thatonly symbols corresponding to the RS packets are passed to softde-mapper 1206.

Soft de-mapper 1206 calculates soft bit metrics using algorithms thatare known in the art including, for example, algorithms described byAkay and Tosato. For correct operation, soft de-mapper 1206 must knowwhich puncture pattern (which trellis code rate) was used in thetransmitter and also the alignment of that pattern with the receivedbits. This information 1221 is provided by frame-sync module 1220 whichdecodes the mode information and also provides a repeating frame syncsignal to which the puncture pattern is aligned, regardless of thecurrent mode. These soft bit metrics are fed to Viterbi decoder 1208that operates in a manner known in the art to arrive at estimates of thebits that were input to the PTCM encoder in the transmitter.

De-randomizer 1213, byte de-interleaver 1214, and RS decoder 1216, whichare all synchronized by the frame-sync signal 1221, respectivelyde-randomize, de-interleave, and decode the byte data to obtain the datathat originally entered the RS encoder in the transmitter.

System Description

Turning now to FIG. 13, certain embodiments of the invention employ aprocessing system that includes at least one computing system 1300deployed to perform certain of the steps described above. Computingsystems may include one or more commercially available systems thatexecutes commercially available operating systems such as MicrosoftWindows®, UNIX or a variant thereof, Linux, a real time operating systemand or a proprietary operating system. The architecture of the computingsystem may be adapted, configured and/or designed for integration in theprocessing system, for embedding in one or more of an image capturesystem, a manufacturing/machining system, a graphics processingworkstation and/or a surgical system or other medical management system.In one example, one or more general purpose (e.g. reduced instructionset computing, or “RISC”) processing cores and/or digital signalprocessing cores are provided in a configurable device. Computing system1300 typically comprises a bus 1302 and/or other mechanisms forcommunicating between processors, whether those processors are integralto the computing system 130 (e.g. 1304, 1305) or located in different,perhaps physically separated computing systems 1300.

Computing system 1300 also typically comprises memory 1306 that mayinclude one or more of random access memory (“RAM”), static memory,cache, flash memory and any other suitable type of storage device thatcan be coupled to bus 1302. Memory 1306 can be used for storinginstructions and data that can cause one or more of processors 1304 and1305 to perform a desired process. Main memory 1306 may be used forstoring transient and/or temporary data such as variables andintermediate information generated and/or used during execution of theinstructions by processor 1304 or 1305. Computing system 1300 alsotypically comprises non-volatile storage such as read only memory(“ROM”) 1308, flash memory, memory cards or the like; non-volatilestorage may be connected to the bus 1302, but may equally be connectedusing a high-speed universal serial bus (USB), Firewire or other suchbus that is coupled to bus 1302. Non-volatile storage can be used forstoring configuration, and other information, including instructionsexecuted by processors 1304 and/or 1305. Non-volatile storage may alsoinclude mass storage device 1310, such as a magnetic disk, optical disk,flash disk that may be directly or indirectly coupled to bus 1302 andused for storing instructions to be executed by processors 1304 and/or1305, as well as other information.

Computing system 1300 may be embodied in audio visual equipment and/ormay be configured to support a display system 1312, such as an LCD flatpanel display, including touch panel displays, electroluminescentdisplay, plasma display, cathode ray tube or other display device thatcan be configured and adapted to receive and display video images andother information. Display 1312 may be provided as a remote terminal orin a session on a different computing system 1300 such that video imagesmay be relayed by, for example, the Internet 1328 or other network 1322.In one example, a digital high definition monitor 1312 may be providedthat processes and displays image data transmitted from a camera. Aninput device 1314 is generally provided locally or through a remotesystem and typically provides for alphanumeric input as well as cursorcontrol 1316 input, such as a mouse, a trackball, etc. It will beappreciated that input and output can be provided to a wireless devicesuch as a PDA, a tablet computer or other system suitable equipped todisplay the images and provide user input.

According to one embodiment of the invention, various functionalelements of a security system may be performed by computing system 1300,including signal processing, control and recording. Processor 1304executes one or more sequences of instructions. For example, suchinstructions may be stored in main memory 1306, having been receivedfrom a computer-readable medium such as storage device 1310. Executionof the sequences of instructions contained in main memory 1306 causesprocessor 1304 to perform process steps according to certain aspects ofthe invention. In certain embodiments, functionality may be provided byembedded computing systems that perform specific functions wherein theembedded systems employ a customized combination of hardware andsoftware to perform a set of predefined tasks. In one example, one ormore multi-core chips may be configured to perform certain of thefunctions described herein. A RISC core may operate to schedule tasks,monitor input and output and to perform other administrative operationswhile digital signal processing components handle the various encoding,decoding, filtering and other processes. Embodiments of the inventionare not limited to any specific combination of hardware circuitry andsoftware.

The term “computer-readable medium” is used to define any medium thatcan store and provide instructions and other data to processor 1304and/or 1305, particularly where the instructions are to be executed byprocessor 1304 and/or 1305 and/or other peripheral of the processingsystem. Such medium can include non-volatile storage, volatile storageand transmission media. Non-volatile storage may be embodied on mediasuch as optical or magnetic disks, including DVD, CD-ROM and BluRay.Storage may be provided locally and in physical proximity to processors1304 and 1305 or remotely, typically by use of network connection.Non-volatile storage may be removable from computing system 1304, as inthe example of BluRay, DVD or CD storage or memory cards or sticks thatcan be easily connected or disconnected from a computer using a standardinterface, including USB, etc. Thus, computer-readable media can includefloppy disks, flexible disks, hard disks, magnetic tape, any othermagnetic medium, CD-ROMs, DVDs, BluRay, any other optical medium, punchcards, paper tape, any other physical medium with patterns of holes,RAM, PROM, EPROM, FLASH/EEPROM, any other memory chip or cartridge, orany other medium from which a computer can read.

Transmission media can be used to connect elements of the processingsystem and/or components of computing system 1300. Such media caninclude twisted pair wiring, coaxial cables, copper wire and fiberoptics. Transmission media can also include wireless media such asradio, acoustic and light waves. In particular radio frequency (RF),fiber optic and infrared (IR) data communications may be used.

Various forms of computer readable media may participate in providinginstructions and data for execution by processor 1304 and/or 1305. Forexample, the instructions may initially be retrieved from a magneticdisk of a remote computer and transmitted over a network or modem tocomputing system 1300. The instructions may optionally be stored in adifferent storage or a different part of storage prior to or duringexecution.

Computing system 1300 may include a communication interface 1318 thatprovides two-way data communication over a network 1320 that can includea local network 1322, a wide area network or some combination of thetwo. For example, an integrated services digital network (ISDN) may usedin combination with a local area network (LAN). In another example, aLAN may include a wireless link. Network link 1320 typically providesdata communication through one or more networks to other data devices.For example, network link 1320 may provide a connection through localnetwork 1322 to a host computer 1324 or to a wide are network such asthe Internet 1328. Local network 1322 and Internet 1328 may both useelectrical, electromagnetic or optical signals that carry digital datastreams. T

Computing system 1300 can use one or more networks to send messages anddata, including program code and other information. In the Internetexample, a server 1330 might transmit a requested code for anapplication program through Internet 1328 and may receive in response adownloaded application that provides for the anatomical delineationdescribed in the examples above. The received code may be executed byprocessor 1304 and/or 1305.

Additional Descriptions of Certain Aspects of the Invention

The foregoing descriptions of the invention are intended to beillustrative and not limiting. For example, those skilled in the artwill appreciate that the invention can be practiced with variouscombinations of the functionalities and capabilities described above,and can include fewer or additional components than described above.Certain additional aspects and features of the invention are further setforth below, and can be obtained using the functionalities andcomponents described in more detail above, as will be appreciated bythose skilled in the art after being taught by the present disclosure.

Certain embodiments of the invention provide signaling systems andmethods that can be employed in security systems. Some of theseembodiments comprise a convolutional byte interleaver that interleaves aframe of data, wherein the interleaver is synchronized to a framestructure. Some of these embodiments comprise a randomizer configured toproduce a randomized data frame from the interleaved data frame. Some ofthese embodiments comprise a punctured trellis code modulator operatedat a selectable code rate that produces a trellis coded data frame fromthe randomized data frame. Some of these embodiments comprise a QAMmapper that maps groups of bits in the trellis coded data frame tomodulation symbols, thereby providing a mapped frame. Some of theseembodiments comprise a synchronizer that adds a synchronization packetto the mapped frame. In some of these embodiments, the punctured trelliscode modulator is bypassed to obtain an optimized net bit rate based ona measured white noise performance of the system. In some of theseembodiments, the same synchronization packet is added to each of asequence of subsequent mapped frames. In some of these embodiments, thesame synchronization packet is added to each mapped frame. In some ofthese embodiments, a portion of the synchronization packet comprises 127symbols.

In some of these embodiments, a portion of the synchronization packetcomprises different binary sequence for real and imaginary parts of themodulation symbols. In some of these embodiments, a portion of thesynchronization packet comprises an identical binary sequence for bothreal and imaginary parts of the modulation symbols. In some of theseembodiments, the synchronization packet comprises data that indicates atransmission mode for the mapped frame. In some of these embodiments,the indication of transmission mode includes a selected QAMconstellation and a selected trellis code rate. In some of theseembodiments, the system generates a constant integral number ofReed-Solomon packets for each frame of data regardless of transmissionmode. In some of these embodiments, the system generates a variableinteger number of modulation symbols for each frame of data regardlessof transmission mode. In some of these embodiments, the system generatesan integral number of puncture pattern cycles per frame of dataregardless of transmission mode.

Certain embodiments of the invention provide a framing method for avariable net bit rate digital communications system that is performed byone or more processors in a modem. Some of these embodiments compriseexecuting one or more instructions that cause the one or more processorsto provide a set of different quadrature amplitude modulation (QAM)constellations. Some of these embodiments comprise executing one or moreinstructions that cause the one or more processors to generate frames ofdata packets using punctured trellis code combinations. In some of theseembodiments, each combination corresponds to an associated mode. Some ofthese embodiments comprise executing one or more instructions that causethe one or more processors to provide a frame having a variable integralnumber of QAM symbols. In some of these embodiments, the number of QAMsymbols corresponds to a selected mode. In some of these embodiments, anassociated number of bytes and Reed-Solomon packets per frame isconstant. In some of these embodiments, the modem simultaneouslytransmits analog and digital representations of a video signal.

Certain embodiments of the invention provide framing methods for avariable net bit rate digital communications system. Some of theseembodiments comprise providing a set of different quadrature amplitudemodulation (QAM) constellations. Some of these embodiments comprisegenerating frames of data packets using punctured trellis codecombinations. In some of these embodiments, each combinationcorresponding to an associated mode. Some of these embodiments compriseproviding a frame having a variable integral number of QAM symbols. Insome of these embodiments, the number of QAM symbols corresponds to aselected mode. In some of these embodiments, wherein an associatednumber of bytes and Reed-Solomon packets per frame is constant. In someof these embodiments, generating frames of data packets using puncturedtrellis code combinations includes generating an integral number ofpuncture pattern cycles per frame of data regardless of the associatedmode. In some of these embodiments, the number of data bits per QAMsymbol for one or more modes is fractional. In some of theseembodiments, a number of trellis coder puncture pattern cycles per frameis an integer for all modes.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be evident to one of ordinaryskill in the art that various modifications and changes may be made tothese embodiments without departing from the broader spirit and scope ofthe invention. Accordingly, the specification and drawings are to beregarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. An apparatus comprising: a QAM demapper thatextracts a trellis coded data frame from modulation symbols in a signalreceived by a modem using transmission mode information provided in thesignal, the transmission mode information comprising a trellis code rateand a QAM constellation used to produce the trellis coded data frame,wherein the transmission mode information is provided in asynchronization packet that comprises different binary sequences forreal and imaginary parts of the modulation symbols; and a Viterbidecoder that extracts a frame comprising a plurality of Reed-Solomon(RS) packets from the trellis coded data frame using the trellis coderate, wherein each of a sequence of frames extracted by the Viterbidecoder comprises a constant number of RS packets regardless oftransmission mode of the each frame.
 2. The apparatus of claim 1,wherein each of the sequence of frames extracted by the Viterbi decodercomprises a variable integral number of QAM symbols.
 3. The apparatus ofclaim 1, wherein a portion of the synchronization packet comprises anidentical binary sequence for both the real part and the imaginary partof the modulation symbols.
 4. A method for variable net bit rate digitalcommunications comprising: extracting a trellis coded data frame frommodulation symbols in a signal received by a modem using transmissionmode information provided in the signal, the transmission modeinformation comprising a trellis code rate and a QAM constellation usedto produce the trellis coded data frame, wherein the transmission modeinformation is provided in a synchronization packet that comprisesdifferent binary sequences for real and imaginary parts of themodulation symbols; and extracting a sequence of frames, each framecomprising a constant number of Reed-Solomon (RS) packets from thetrellis coded data frame using the trellis code rate regardless oftransmission mode of the each frame.
 5. The method of claim 4, whereineach of the sequence of frames comprises a variable integral number ofQAM symbols.
 6. The method of claim 4, wherein a portion of thesynchronization packet comprises an identical binary sequence for boththe real part and the imaginary part of the modulation symbols.
 7. Anon-transitory computer-readable medium having instructions storedthereon, wherein the instructions when executed by one or moreprocessors of a processing circuit cause the processing circuit to:extract a trellis coded data frame from modulation symbols in a signalreceived by a modem using transmission mode information provided in thesignal, the transmission mode information comprising a trellis code rateand a QAM constellation used to produce the trellis coded data frame,wherein the transmission mode information is provided in asynchronization packet that comprises different binary sequences forreal and imaginary parts of the modulation symbols; and extract asequence of frames, each frame comprising a constant number ofReed-Solomon (RS) packets from the trellis coded data frame using thetrellis code rate regardless of transmission mode of the each frame. 8.The computer-readable medium of claim 7, wherein each of the sequence offrames comprises a variable integral number of QAM symbols.
 9. Thecomputer-readable medium of claim 7, wherein a portion of thesynchronization packet comprises an identical binary sequence for boththe real part and the imaginary part of the modulation symbols.